Walker's plan to strip most public workers of nearly all their union rights sparked massive protests at the Capitol in 2011. A small group of protesters still gathers in the building's rotunda daily to sing and chant against Walker.

The Capitol Police Department, an agency Walker controls, began cracking down on the singers in September by issuing scores of citations to group leaders, mostly for failing to obtain permits.

The lawsuit filed Monday alleges the police policy is overbroad and violates the protesters' right to free expression.